Known sensors of electric conductivity for said kind of application are, for example the apparatus known as "ADSV" from the USSR Inventor's Certificate No. 527,640. These known sensors, however, have the drawback that their design cannot provide an arrangement capable of orienting the fibers as to their length with respect to the inlet opening of the sensor means. The speed of the suspension in the opening of the sensor means is substantially higher than in the close neighbourhood of the sensor means, causing differences of speed of the carrying medium of the suspension and of the fibers. The sensor means of the mentioned apparatus "ADSV" is submersible and does not enable a sucking off of the measured suspension without a residuum, which represents a drawback where a defined weight of fibers in a suspension has to be measured.
Another known device is the KAJAANI FS-100 Optical Fibre Length Analyzer which operates on a different principle. The FS-100 has a capillary tube and an optical system monitoring the passage of fibres over the active analyzer portion. The analyzer portion consists of a light source, polarized light filters, optical elements, and a light sensitive sensor. A suspension is sucked into the capillary tube by vacuum. The shape of the tube enables the fibers to be oriented in a longitudinal direction. Light emitted from the source disposed at one side of the capillary tube passes through the polarized light filter, capillary, another polarized light filter, and finally impinges on the light sensitive sensor. Due to a change in polarized light provoked by the presence of fiber in the capillary tube, a fiber image is formed on the light sensitive sensor by means of the optical elements. The light sensitive sensor has a plurality of sections and, depending on the number of sections activated, the number and length of fibers is determined by an electronic circuit. This arrangement is rather complex and expensive.